Anupam K. Misra

A combined remote Raman and LIBS instrument for characterizing minerals with 532 nm laser excitation.

The authors have developed an integrated remote Raman and laser-induced breakdown spectroscopy (LIBS) system for measuring both the Raman and LIBS spectra of minerals with a single 532 nm laser line of 35 mJ/pulse and 20 Hz. The instrument has been used for analyzing both Raman and LIBS spectra of carbonates, sulfates, hydrous and anhydrous silicates, and iron oxide minerals in air. These experiments demonstrate that by focusing a frequency-doubled 532 nm Nd:YAG pulsed laser beam with a 10x beam expander to a 529-microm diameter spot on a mineral surface located at 9 m, it is possible to measure simultaneously both the remote Raman and LIBS spectra of calcite, gypsum and olivine by adjusting the laser power electronically. The spectra of calcite, gypsum, and olivine contain fingerprint Raman lines; however, it was not possible to measure the remote Raman spectra of magnetite and hematite at 9 m because of strong absorption of 532 nm laser radiation and low intensities of Raman lines from these minerals. The remote LIBS spectra of both magnetite and hematite contain common iron emission lines but show difference in the minor amount of Li present in these two minerals. Remote Raman and LIBS spectra of a number of carbonates, sulfates, feldspars and phyllosilicates at a distance of 9 m were measured with a 532-nm laser operating at 35 mJ/pulse and by changing photon flux density at the sample by varying the spot diameter from 10 mm for Raman to 530 microm for LIBS measurements. The complementary nature of these spectra is highlighted and discussed. The combined Raman and LIBS system can also be re-configured to perform micro-Raman and micro-LIBS analyses, which have applications in trace/residue analysis and analysis of very small samples in the nano-gram range.

Near-Infrared Micro-Raman Spectroscopy for in Vitro Detection of Cervical Cancer

Near-infrared Raman spectroscopy is a powerful analytical tool for detecting critical differences in biological samples with minimum interference in the Raman spectra from the native fluorescence of the samples. The technique is often suggested as a potential screening tool for cancer. In this article we report in vitro Raman spectra of squamous cells in normal and cancerous cervical human tissue from seven patients, which have good signal-to-noise ratio and which were found to be reproducible. These preliminary results show that several Raman features in these spectra could be used to distinguish cancerous cervical squamous cells from normal cervical squamous cells. In general, the Raman spectra of cervical cancer cells show intensity differences compared to those of normal squamous cell spectra. For example, several well-defined Raman peaks of collagen in the 775 to 975 cm−1 region are observed in the case of normal squamous cells, but these are below the detection limit of normal Raman spectroscopy in the spectra of invasive cervical cancer cells. In the high frequency 2800 to 3100 cm−1 region, it is found that the peak area under the CH stretching band is lower by a factor of approximately six in the spectra of cervical cancer cells as compared with that of the normal cells. The Raman chemical maps of regions of cancer and normal cells in the cervical epithelium made from the spectral features in the 775 to 975 cm−1 and 2800 to 3100 cm−1 regions are also found to show good correlation with each other.

Remote pulsed raman spectroscopy of inorganic and organic materials to a radial distance of 100 meters

A portable pulsed remote Raman spectroscopy system has been fabricated and tested to 100 m radial distance. The remote Raman system is based on a directly coupled f/2.2 spectrograph with a small (125 mm diameter) telescope and a frequency-doubled Nd:YAG pulsed laser (20 Hz, 532 nm, 25 mJ/pulse) used as the excitation source in a co-axial geometry. The performance of the Raman system is demonstrated by measuring the gated Raman spectra of calcite, sodium phosphate, acetone, and naphthalene. Raman spectra of these materials were recorded with the 532 nm pulsed laser excitation and accumulating the spectra with 600 laser shots (30 s integration time) at 100 m with good signal-to-background ratio. The remote pulsed Raman system can be used for remotely identifying both inorganic and organic materials during daytime or nighttime. The system will be useful for terrestrial applications such as monitoring environmental pollution and for detecting minerals and organic materials such as polycyclic aromatic hydrocarbons (PAHs) on planetary surfaces such as Mars.

Single-pulse standoff Raman detection of chemicals from 120 m distance during daytime.

The capability to analyze and detect the composition of distant samples (minerals, organics, and chemicals) in real time is of interest for various fields including detecting explosives, geological surveying, and pollution mapping. For the past 10 years, the University of Hawaii has been developing standoff Raman systems suitable for measuring Raman spectra of various chemicals in daytime or nighttime. In this article we present standoff Raman spectra of various minerals and chemicals obtained from a distance of 120 m using single laser pulse excitation during daytime. The standoff Raman system utilizes an 8-inch Meade telescope as collection optics and a frequency-doubled 532 nm Nd: YAG laser with pulse energy of 100 mJ/pulse and pulse width of 10 ns. A gated intensified charge-coupled device (ICCD) detector is used to measure time-resolved Raman spectra in daytime with detection time of 100 ns. A gate delay of 800 ns (equivalent to target placed at 120 m distance) was used to minimize interference from the atmospheric gases along the laser beam path and near-field scattering. Reproducible, good quality single-shot Raman spectra of various inorganic and organic chemicals and minerals such as ammonium nitrate, potassium perchlorate, sulfur, gypsum, calcite, benzene, nitrobenzene, etc., were obtained through sealed glass vials during daytime. The data indicate that various chemicals could easily be identified from their Raman fingerprint spectra from a far standoff distance in real time using single-shot laser excitation.

Time-resolved remote Raman study of minerals under supercritical CO2 and high temperatures relevant to Venus exploration

We report time-resolved (TR) remote Raman spectra of minerals under supercritical CO2 (approx. 95 atm pressure and 423 K) and under atmospheric pressure and high temperature up to 1003 K at distances of 1.5 and 9 m, respectively. The TR Raman spectra of hydrous and anhydrous sulphates, carbonate and silicate minerals (e.g. talc, olivine, pyroxenes and feldspars) under supercritical CO2 (approx. 95 atm pressure and 423 K) clearly show the well-defined Raman fingerprints of each mineral along with the Fermi resonance doublet of CO2. Besides the CO2 doublet and the effect of the viewing window, the main differences in the Raman spectra under Venus conditions are the phase transitions, the dehydration and decarbonation of various minerals, along with a slight shift in the peak positions and an increase in line-widths. The dehydration of melanterite (FeSO4 · 7H2O) at 423 K under approximately 95 atm CO2 is detected by the presence of the Raman fingerprints of rozenite (FeSO4 · 4H2O) in the spectrum. Similarly, the high-temperature Raman spectra under ambient pressure of gypsum (CaSO4 · 2H2O) and talc (Mg3Si4O10(OH)2) indicate that gypsum dehydrates at 518 K, but talc remains stable up to 1003 K. Partial dissociation of dolomite (CaMg(CO3)2) is observed at 973 K. The TR remote Raman spectra of olivine, α-spodumene (LiAlSi2O6) and clino-enstatite (MgSiO3) pyroxenes and of albite (NaAlSi3O8) and microcline (KAlSi3O8) feldspars at high temperatures also show that the Raman lines remain sharp and well defined in the high-temperature spectra. The results of this study show that TR remote Raman spectroscopy could be a potential tool for exploring the surface mineralogy of Venus during both daytime and nighttime at short and long distances.

Remote Raman spectroscopic detection of minerals and organics under illuminated conditions from a distance of 10 m using a single 532 nm laser pulse.

Raman spectra of several minerals and organics were obtained from a small portable instrument at a distance of 10 m in a well-illuminated laboratory with a single 532 nm laser pulse with energy of 35 mJ/pulse. Remote Raman spectra of common minerals (dolomite, calcite, marble, barite, gypsum, quartz, anatase, fluorapatite, etc.) obtained in a short period of time (1.1 μs) clearly show Raman features that can be used as fingerprints for mineral identification. Raman features of organics (benzene, cyclohexane, 2-propanol, naphthalene, etc.) and other chemicals such as oxides, silicates, sulfates, nitrates, phosphates, and carbonates were also easily detected. The ability to identify minerals from their Raman spectra obtained from a single laser pulse has promise for future space missions where power consumption is critical. Such a system could be reduced in size by minimizing the cooling requirements for the laser unit. The remote Raman system is also capable of performing time-resolved measurements. Data indicate that further improvement in the performance of the system is possible by reducing the gate width of the detector (ICCD) from 1.1 μs to approximately 20 ns, which would significantly reduce the background signal from daylight or a well-illuminated laboratory. The 1.1 μs signal gating was effective in removing nearly all background fluorescence with 532 nm excitation, indicating that the fluorescence in most minerals is probably from long-lifetime inorganic phosphorescence.

Novel Micro-Cavity Substrates for Improving the Raman Signal from Submicrometer Size Materials

A novel and simple method for improving the detection limit of conventional Raman spectra using a micro-Raman system and picoliter volumes is presented. A micro-cavity in a reflecting metal substrate uses various mechanisms that collectively improve the entire Raman spectrum from the sample. A micro-cavity with a radius of several micrometers acts as a very effective device that provides multiple excitation of the sample with the laser and couples the forward-scattered Raman photons toward the collection optics in the back-scattered Raman geometry. One of the important features of the micro-cavity substrate is that it enhances the entire Raman spectrum of the molecules under investigation and maintains the relative intensity ratios of the various Raman bands. This feature of maintaining the overall integrity of the Raman features during signal enhancement makes the micro-cavity substrate ideal for forensic science applications for chemical detection of residual traces and other applications requiring low sample concentrations. The spectra measured in these cavities are also observed to be highly reproducible and reliable. A simple method for fabricating micro-cavity substrates with precise sizes and shapes is described. It is further shown that micro-cavities coated with nanofilms of gold take advantage of both surface-enhanced Raman scattering (SERS) and micro-cavity methods and also significantly improve sample detection limits.

1⧸ƒ noise power measurements on Tl2Ba2Can−1CunO4+2n (n = 2 and 3)

Abstract We have performed systematic 1⧸ƒ noise power measurements on bulk Tl 2 Ba 2 Ca n −1 Cu n O 4+2 n ( n = 2 and 3) materials normal and superconducting states. The 1⧸ƒ noise power is generally found to decrease with temperature until the superconducting transition region is approached. We observe no correlation between the magnitude of the 1⧸ƒ noise power in the normal state and the sample quality. In contrast to bulk Y 1 Ba 2 Cu 3 O 7− δ samples, the enhancement of the 1⧸ƒ noise in the superconducting transiti on region is reduced or even totally disappears in some of our good quality samples. We discuss a model calculation based on potential fluctuations in the grain boundary regions modeled as metal-insulator-metal junctions. We demonstrate that if direct knowledge of the barrier characteristics of the junctions is available, it is possible to explain our experimental data quantitstively.

Planetary Geochemical Investigations Using Raman and Laser-Induced Breakdown Spectroscopy

An integrated Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) instrument is a valuable geoanalytical tool for future planetary missions to Mars, Venus, and elsewhere. The ChemCam instrument operating on the Mars Curiosity rover includes a remote LIBS instrument. An integrated Raman-LIBS spectrometer (RLS) based on the ChemCam architecture could be used as a reconnaissance tool for other contact instruments as well as a primary science instrument capable of quantitative mineralogical and geochemical analyses. Replacing one of the ChemCam spectrometers with a miniature transmission spectrometer enables a Raman spectroscopy mineralogical analysis to be performed, complementing the LIBS chemical analysis while retaining an overall architecture resembling ChemCam. A prototype transmission spectrometer was used to record Raman spectra under both Martian and Venus conditions. Two different high-pressure and high-temperature cells were used to collect the Raman and LIBS spectra to simulate surface conditions on Venus. The resulting LIBS spectra were used to generate a limited partial least squares Venus calibration model for the major elements. These experiments demonstrate the utility and feasibility of a combined RLS instrument.

The pressures and temperatures of meteorite impact: Evidence from micro-Raman mapping of mineral phases in the strongly shocked Taiban ordinary chondrite

Abstract Taiban is a heavily shocked L6 chondrite showing opaque melt veins. Raman spectroscopy was used to characterize the high-pressure silicate assemblages in a thin section crossed by a shock-created 4 mm wide melt vein. Raman spectra using different excitation wavelengths allowed identification of mineral phases such as olivine, wadsleyite, ringwoodite, high-Ca clinopyroxene, majorite-pyrope, jadeite, maskelynite, and lingunite. Olivine is Fe depleted in contact with the ringwoodite, which suggests chemical fractionation during a solid-state olivine-ringwoodite transformation. Raman imaging revealed a close correlation between the blue ringwoodite color and the peak observed at 877 cm-1; this signal shows strong near-resonance Raman enhancement when measured with near-IR excitation lines (785 and 830 nm) close to the optical absorption bands of the ringwoodite. We propose that the blue color of the ringwoodite is due to a small amount of iron in fourfold coordination inside the spinel structure, and that yields the observed spectral features in differently colored ringwoodite. Under the formation conditions of the studied silicate pocket, all enstatite transformed to a majorite-pyrope solid solution, whereas the high-Ca clinopyroxene likely remained unchanged. Maskelynite grains in the margins of the pocket often contain lingunite or are totally transformed to jadeite. Based on static high-pressure results, the mineral assemblages in the pocket suggest peak pressure in the 17-20 GPa range with maximum temperature (Tmax) in the range 1850-1900 K as the formation conditions for this Taiban chondrite during shock.